Various kinds of X-ray microscopes are known which differ more or less with respect to the following: the optical configuration with respect to the beam source used, the optics for focussing the X-ray beam on the specimen to be investigated and the optics for imaging the specimen on the X-ray detector used to provide the image.
X-ray microscopes are described, for example, wherein mirror optics are used for imaging the specimen on the detector such as a Wolter optic which images the specimen with a grazing incidence of the X-radiation. The quality of the microscopic image generated with such microscopes is however not especially good since considerable imaging errors are associated with the mirror optics. In mirror optics operating with grazing incidence, the image error associated therewith is the so-called angle-tangent error. These image errors limit in principle the possible resolution which can be obtained with the microscope and is pregiven by the aperture of the optics.
X-ray microscopes are known wherein so-called zone plates are utilized for focussing the X-radiation on the specimen as well as for imaging the specimen on the detector. These zone plates make it possible (similar to very thin lenses) to provide an imaging of the object or specimen which is free of image faults and therefore of high resolution. However, the zone plates have a significantly less efficiency than mirror optics. The efficiency in practice lies between 5% and 15%, that is, a maximum of only 15% of the X-radiation impinging on the zone plate is utilized for imaging.
An overview of the various X-ray microscopes is provided in the text of D. Rudolph et al entitled "X-Ray Microscopy", Volume 43 (1984) and published by Springer.
Starting on page 192 of this text, an X-ray microscope is described wherein the condenser as well as the objective are configured as zone plates. The zone plate used as the condenser not only focusses the X-radiation on the object but also functions as a monochromator and separates the monochromatic radiation required for the high resolution imaging from the more or less expanded wavelength range supplied by the X-ray source. This takes place simply by a suitable pin-hole diaphragm on the optical axis which effects the condition that only one of the monochromatic images passes through the diaphragm with the image arising on the optical axis as a consequence of the wavelength dependency of the focal width of the zone plate.
The X-ray microscope described above is relatively light attenuating with the above-mentioned low efficiency because of the use of zone plates so that long exposure times result which can lead to motional blurring during exposure when taking recordings of living cells. For this reason, one is dependent upon the most intensive X-radiation sources.
For the reasons given above, synchroton radiation from electron storage rings is used almost exclusively. However, this brings with it the disadvantage that the X-ray microscope is not self-contained, that is, the user is tied to the few electron storage rings with respect to the measuring time which is available.
So-called plasma focus sources are also known as X-radiation sources. Such X-ray sources are described for example in U.S. Pat. No. 4,596,030 and do not however continuously supply X-radiation; instead, they supply short X-ray pulses which are followed by a relatively long dead time during which the capacitors of the X-radiation sources must be recharged. The X-radiation contained in one pulse is in many cases inadequate.